Raman spectroscopy is an effective tool for material characterization and identification, in which monochromatic light (usually laser light) interacts with molecular vibrations, phonons, or other excitations in the material, resulting in a frequency shift of the laser light. This frequency shift reveals information about the composition of the material. In an inhomogeneous material (e.g. biological tissue) which has spatial variation in its optical properties, the penetration depth of the laser light is very limited due to a high scattering loss. As a result, the application of Raman spectroscopy has conventionally been restricted to surface analysis. To analyze substance that lies below the surface layer of an inhomogeneous material, one has to increase the penetration depth of laser light in the highly scattering medium.